Add Google OAuth setup guide and integrate OAuth functionality into backend and frontend

Commit notes

- Introduced `GOOGLE_OAUTH_SETUP.md` to provide a comprehensive guide for setting up Google OAuth in the BeatStep application. - Updated backend configuration to include Google OAuth credentials and routes for handling OAuth login and callback. - Created `OAuthHandler` to manage Google OAuth flow, including user authentication and token generation. - Modified the User model to support OAuth users with new fields for provider and provider ID. - Enhanced frontend authentication flow with a new `AuthCallbackPage` to handle token reception and error management. - Added Google login button to the authentication page for seamless user experience. - Updated environment variables and database migration instructions to accommodate OAuth integration. - Ensured proper error handling and CSRF protection during the OAuth process.